1. Technical Field
The present invention relates in general to an improved suspension for disk drives and, in particular, to an improved system, method, and apparatus for providing a continuous reference plane in wireless suspensions.
2. Description of the Related Art
Data access and storage systems comprise one or more storage devices that store data on storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, one to six disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm). Hard disk drives have several different typical standard sizes or formats, including server, desktop, mobile and microdrive.
A typical HDD also utilizes an actuator assembly. The actuator moves magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location having an air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive, by a cushion of air generated by the rotating disk. Within most HDDs, the magnetic read/write head transducer is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk. Each slider is attached to the free end of a suspension that in turn is cantilevered from the rigid arm of an actuator. Several semi-rigid arms may be combined to form a single movable unit having either a linear bearing or a rotary pivotal bearing system.
The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop and settle directly over the desired track.
The motor used to rotate the disk is typically a brushless DC motor. The disk is mounted and clamped to a hub of the motor. The hub provides a disk mounting surface and a means to attach an additional part or parts to clamp the disk to the hub. In most typical motor configurations of HDDs, the rotating part of the motor (the rotor) is attached to or is an integral part of the hub. The rotor includes a ring-shaped magnet with alternating north/south poles arranged radially and a ferrous metal backing. The magnet interacts with the motor's stator by means of magnetic forces. Magnetic fields and resulting magnetic forces are induced via the electric current in the coiled wire of the motor stator. The ferrous metal backing of the rotor acts as a magnetic return path.
The suspension of a conventional disk drive typically includes a relatively stiff load beam with a mount plate at the base end, which subsequently attaches to the actuator arm, and whose free end mounts a flexure that carries the slider and its read/write head transducer. Disposed between the mount plate and the functional end of the load beam is a ‘hinge’ that is compliant in the vertical bending direction (normal to the disk surface). The hinge enables the load beam to suspend and load the slider and the read/write head toward the spinning disk surface. It is then the job of the flexure to provide gimbaled support for the slider so that the read/write head can pitch and roll in order to adjust its orientation for unavoidable disk surface run out or flatness variations.
The flexure in a wireless suspension is generally made out of a laminated multilayer material. Typically, it consists of a conductor layer (like copper), a di-electric layer (like polyimide), and a support layer (like stainless steel). The electrical lead lines are etched into the conductor layer, while the polyimide layer serves as the insulator from the underlying steel support layer. The steel support layer is also patterned to provide strength and gimbaling characteristics to the flexure. The conducting leads, called traces, which electrically connect the head transducer to the read/write electronics, are often routed on both sides of the suspension, especially in the gimbal region. Normally the traces consist of copper conductor with polyimide dielectric layer but no support stainless steel layer and only provide the electrical function. The mechanical function is provided by the flexure legs (stainless steel only) which normally run adjacent to the traces.
In disk drives, the rate of data transfer is increasing as users demand faster access to their data. Several factors influence the transfer rate: the magnetic transducer, the transmission line between the magnetic transducer and the arm electronics (A/E), and the A/E itself. The present invention generally addresses improving the transmission line and, in particular, the wireless suspension. Perturbations in the transmission line on wireless suspensions cause reflection points for the data signal as well as increase impedance in the transmission line. A few causes for perturbations are: sharp bends in the conductors, rapid changes in their cross section, and disruptions in the reference plane under the conductors. The present invention is particularly focused on eliminating disruptions in the reference plane under the conductors. The reference plane in wireless suspensions, such as an Integrated Lead Suspension (ILS) and a Circuit Integrated Suspension (CIS), is the steel that is used to fabricate the suspension's flexure.
Disruptions in reference plane are a by-product of fabricating the flexure to its desired shape and function. Referring to FIGS. 14-17, the write driver, which is part of the A/E, is connected to the flexure steel to provide a reference plane to ground. This is a constant electrical return path for the transmission lines and write driver and does not electrically short the conductors. It is desirable to reduce the total impedance (Zo) of the transmission lines in order to achieve higher data rate. Zo is related to the total capacitance and inductance of the transmission lines through the relationship of:Zo=√[L/C]where L is the inductance and C is the capacitance of the transmission lines. By minimizing the discontinuities in the reference plane, L is decreased and C is increased.
In the prior art, some designs such as U.S. Pat. Nos. 5,862,010 and 5,986,853, address the problem described above by adding a second layer of copper in the ILS laminate. One drawback to this approach includes the cost of fabrication, both in the laminate and in the ILS etching process. Producing ILS designs with the desired flexure stiffness also becomes a challenge with an extra layer of copper. Thus, an improved continuous reference plane for wireless suspensions would be desirable.
Referring again to FIGS. 14-17, the write driver in a signal generating system of an HDD generally produces a differential mode (DM) write signal. But there can be also a considerable common mode (CM) signal component. If this CM component is not transmitted, the write driver does not work properly. There is always present an impedance asymmetry in the signal generating system due primarily to the ILS and the write driver. This asymmetry can cause the CM signal component to be reflected and converted into a DM signal. The transmission line model for CM (FIGS. 14 and 15) illustrates that the electrical potential of the two signal lines is in phase; both lines have the same potential. Observation of the electric field illustrates that the signal return path for each conductor is through the reference plane along the full length of the reference plane adjacent to the conductor. The impedance of the system is greatly dependent on the continuity of the reference plane.
The transmission line model for DM (FIGS. 16 and 17) illustrates that the electrical potential of the two signal lines is out of phase. Observations of the electrical field illustrate that the return path for the electric current in the reference plane underneath the signal conductors is local and transverse (perpendicular to the conductors) through the reference plane between conductors. Disruptions or discontinuities in the reference plane for a DM signal affect the impedance of the transmission line to a lesser degree than similar discontinuities in the reference plane for a CM signal. The effect on impedance of a discontinuity for a DM signal is proportional to the length of the discontinuity in the reference plane to the total length of the reference plane adjacent to the conductor. Where as a discontinuity in the reference plane for a CM signal creates a break and is totally disruptive to the signal return path and therefore has a very large effect on impedance. In a well-designed ILS or CIS all factors that affect the transmission lines' impedance are balanced for optimum performance of the signal generating system. This is especially important at high data rates.